This invention is generally related to monolithic coupled-dual resonator crystals and in particular is directed to an improved method and apparatus for accurately measuring the characteristic frequencies of such crystals at any step in the manufacturing process following formation of the resonators.
The disclosed exemplary embodiments have particular utility in the frequency adjustment and testing of monolithic filter crystals. This disclosure is related to and is a continuation-in-part of two earlier applications, Ser. No. 07/480,773, now U.S. Pat. No. 5,049,828 entitled "Method and Apparatus for Parameter Measurement of Coupled-Dual Resonator Crystals" and Ser. No. 07/480,774, now U.S. Pat. No. 5,047,726 entitled "Method and Apparatus for Implementing the Four-Frequency Measuring Process for Coupled-Dual Crystals Using Both Resonator Ports", both filed Feb. 16, 1989.
As indicated in the above noted parent applications, monolithic filter crystal structures and theory of operation, as well as methods of making, testing and adjusting such structures, are well known. The following commonly assigned patents listed below are also generally related to such structure and/or methods.
U.S. Pat. No. 4,093,914--Peppiatt et al (1978) PA1 U.S. Pat. No. 4,433,316--Roberts et al (1984) PA1 U.S. Pat. No. 4,477,952--Crescenzi et al (1984) PA1 U.S. Pat. No. 4,627,379--Roberts et al (1986) PA1 U.S. Pat. No. 4,676,993--Roberts et al (1987) PA1 U.S. Pat. No. 4,833,430--Roberts et al (1989) PA1 U.S. Pat. No. 4,839,618--Roberts et al (1989)
Typically, a monolithic filter crystal is constructed as shown in FIGS. 1, 2A and 2B. It comprises multiple acoustically coupled resonators on a single crystal blank 12. A coupled-dual resonator has an input electrode 14, for example, and an output electrode 16, for example, on one face of the blank and common electrodes, 22, 24 on the opposite face of the blank with a first resonator formed by the input electrode 14 and the common electrode 22, and a second resonator formed by the output electrode 16 and common electrode 24.
FIGS. 2A and 2B show the active and ground sides of a mounted crystal. The ground side may or may not have a gap between the resonators as shown. The equivalent circuit of a typical coupled-dual resonator crystal is shown in FIG. 3.
Monolithic filter crystals are used extensively in the radio communication industry. Exemplary uses include IF filter and discriminator applications in mobile radios and cellular telephones.
In the manufacture of coupled-dual crystals, the major parameters under consideration are the two resonator frequencies and coupling. As indicated in commonly assigned U.S. Pat. No. 4,093,914, which is hereby incorporated by reference, accurate determination of these desired characteristics can be derived mathematically by means of a four-frequency measurement technique. The four frequencies are designated--F1, F2, F3 and F4.
Application Ser. No. 07/480,774 now U.S. Pat. No. 6,047,726, the disclosure of which is incorporated herein, details our discovery of the means of obtaining the four critical frequencies used in the calculation of the key crystal parameters wherein both ports of the coupled-dual crystal are monitored. The common method of displaying the four frequencies of interest is by inserting the crystal in a simple voltage divider type network in which the driving-point impedance whose phase zeros are to be measured is put in the series arm. This apparatus of the noted parent application is shown in FIG. 4.
In normal use, a frequency synthesizer is connected to J1, the reference probe (A) of a vector voltmeter is connected to J2, and the B probe is connected to J3. The first two frequencies, namely F1 and F3, as illustrated in FIG. 5A, are obtained by inserting the crystal with the B port in the series arm with switch S1 closed. Then a band of frequencies is applied at J1 until the B probe of the vector voltmeter indicates zero phase in the vicinity of the two specific maximum voltage amplitudes as also illustrated in FIG. 5A.
After measuring the values of Fl and F3, the crystal is removed and reinserted with the A port in the series arm and switch S1 opened. Frequencies F2 and F4, as illustrated in FIG. 5B, are obtained by using the frequency sweep technique previously described. A network analyzer such as a Hewlett Packard HP 3577A may also be used to obtain the four critical frequencies needed for parameter measurement by means of reflective measurement techniques. Such analyzers are two port measuring instruments which makes it easier and more convenient to switch between the various ports of the crystal as required by the reflective measurement techniques employed.
As may be seen from the above noted prior art, frequency adjustment of monolithic filter crystals is known and is achieved by various means. For example, the most noted adjustment method is a vacuum deposition process whereby silver, gold or other suitable material is deposited through a specially designed mask onto the electrode of the crystal. The deposited material produces a mass loading effect which causes the frequency of the crystal to decrease. Other techniques for frequency adjustment include mass addition by chemical action and mass removal by laser.
The methodology taught by Toliver, Roberts and Crescenzi in commonly assigned U.S. Pat. No. 4,696,993 discloses a method for frequency adjustment of coupled-duals by monitoring only one port and without using compensation on either port. This method worked well when the design of the crystal was such that the four critical frequencies always displayed zero phase crossings. However, in the development of unsymmetrical filters and/or filters at higher operating frequencies, we noticed that the measurement capability began to deteriorate. At still higher frequencies we noticed that measurement of the four critical frequencies using the conventional method and apparatus became unattainable, thus making accurate adjustment of the filter parameters impossible. U.S. patent application Ser. No. 07/480,773, now U.S. Pat. No. 5,049,828, discloses a single port measurement apparatus that solved this problem.
We have discovered a new method and apparatus which conveniently uses a new two-port measurement technique allowing not only coupled-dual crystal parameter measurements but accurate parameter adjustments as well. The exemplary embodiments disclosed exhibit a novel manner of implementing transmission or reflective two-port measuring techniques in production and test systems without requiring the removal of the crystal from the measurement fixture as in the case in 07/480,774, now U.S. Pat. No. 5,047,726. Such embodiments also allow implementing the two-port measurement method with the inclusion of compensation of the driving-point impedance of the filter, as well as using the compensated measuring techniques for selectively changing the plate mass distribution on a coupled-dual crystal to adjust the characteristic frequencies.
An important aspect of the known two-port measuring technique is that it allows measurement of the tuning frequency of each resonator, wherein the tuning frequency of either of the two resonators is the lower frequency of the two phase zeros of the input impedance to the resonator with the other resonator either open or short circuited to the common lead. A further object of our embodiments is to obtain a directly measurable response for each resonator which provides more flexibility when using the four-frequency measurement method for frequency adjustment. That is to say, during frequency adjustment where only moderate accuracy is required, in the interest of speed only the tuning frequencies need to be monitored.
In this regard, by monitoring only the tuning frequencies with our two-port embodiments, which may include selectively compensated driving-point impedance features, gross imbalances in the resonator frequencies can be more easily detected and resolved before the final frequency adjustment. Commonly assigned U.S. Pat. No. 4,676,993 discloses a method and apparatus for selectively fine tuning a coupled-dual resonator crystal in conjunction with the measuring technique taught by U.S. Pat. No. 4,093,914. Use of our two-port compensated or uncompensated embodiments for detecting the aforementioned gross imbalances allows considerable enhancement of the cited prior art fine tuning system in realizing desired filter characteristics.
Alternatively, when very accurate measurement of the filter crystal parameters, such as frequencies F1, F2, F3 and F4 are required, our embodiments can be converted to a four-frequency measurement technique with measurements made in the manner taught, for example, in application Ser. Nos. 07/480,773, now U.S. Pat. No. 5,049,828 or 07/480,774, now U.S. Pat. No. 5,047,726.